Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract infections in infants and small children. RSV infection is characterized by significant immunopathology, which is mediated by CD8+ cytotoxic T lymphocytes (CTL). Past studies have suggested that the multiple mechanisms employed by CTLs to eliminate cells may have differential effects on viral clearance and RSV-associated illness. This study seeks to define the contributions of the different CTL effector mechanisms and CD25+ T regulatory cells to the clearance of primary RSV infection, as well as their relative impact on the virus-associated immunopathology.